Embolic protection device

ABSTRACT

A collapsible filter element for a transcatheter embolic protection device, the filter element comprises a collapsible filter body of polymeric material which is movable between a collapsed stored position for movement through a vascular system and an expanded position for extension across a blood vessel such that blood passing through the blood vessel is delivered through the filer element. A proximal inlet portion of the filter body has one or more inlet openings sized to allow blood and embolic material enter the filter body. A distal outlet portion of the filter body has a plurality of generally circular outlet openings sized to allow through-passage of blood, but to retain embolic material within the filter body. The distal outlet portion of the filter body in the region of the outlet openings has means for reducing shear stress on blood passing through the outlet openings. The shear stress reducing means includes lead-in and lead-out radiussed portions of the filter body leading to the outlet holes. The porosity of the distal portion of the filter body decreases towards the distal end. A blind portion extends for at least 5% of the length of the body. Preferably there are between 200 and 300 outlet opening with an average diameter of approximately 150 microns.

This is a request for a Continuation Application of prior applicationSer. No. 10/689,846 filed Oct. 22, 2003 now abandoned, which is aContinuation of application Ser. No. 09/986,064 (now U.S. Pat. No.6,726,701), filed Nov. 7, 2001, which is a continuation ofPCT/IE00/00055 filed May 8, 2000, which claims benefit of priority fromIrish Patent Applications Nos. PCT/IE99/00033 and PCT/IE99/00036, filedMay 7, 1999. The entire disclosures of the prior applications,application Ser. Nos. 10/689,846; 09/986,064; and PCT/IE00/00055 areconsidered part of the disclosure of the accompanying continuationapplication and are hereby incorporated by reference.

INTRODUCTION

The term “STROKE” is used to describe a medical event whereby bloodsupply to the brain or specific areas of the brain is restricted orblocked to the extent that the supply is inadequate to provide therequired flow of oxygenated blood to maintain function. The brain willbe impaired either temporarily or permanently, with the patientexperiencing a loss of function such as sight, speech or control oflimbs. There are two distinct types of stroke, haemorrhagic and embolic.This invention addresses embolic stroke.

Medical literature describes caroitid artery disease as a significantsource of embolic material. Typically, an atherosclerotic plaque buildsup in the carotid arteries. The nature of the plaque variesconsiderably, but in a significant number of cases pieces of the plaquecan break away and flow distally and block bloodflow to specific areasof the brain and cause neurological impairment. Treatment of the diseaseis classically by way of surgical carotid endarterectomy whereby, thecarotid artery is cut and the plaque is physically removed from thevessel. The procedure has broad acceptance with neurologicalcomplication rates quoted as being low, somewhere in the order of 6%although claims vary widely on this.

Not all patients are candidates for surgery. A number of reasons mayexist such that the patients could not tolerate surgical intervention.In these cases and an increasing number of candidates that are surgicalcandidates are being treated using transcatheter techniques. In thiscase, the evolving approach uses devices inserted in the femoral arteryand manipulated to the site of the stenosis. A balloon angioplastycatheter is inflated to open the artery and an intravascular stent issometimes deployed at the site of the stenosis. The action of thesedevices as with surgery can dislodge embolic material which will flowwith the arterial blood and if large enough, eventually block a bloodvessel and cause a stroke.

It is known to permanently implant a filter in human vasculature tocatch embolic material. It is also known to use a removable filter forthis purpose. Such removable filters typically comprise umbrella typefilters comprising a filter membrane supported on a collapsible frame ona guidewire for movement of the filter membrane between a collapsedposition against the guidewire and a laterally extending positionoccluding a vessel. Examples of such filters are shown in U.S. Pat. No.4,723,549, U.S. Pat. No. 5,053,008, U.S. Pat. No. 5,108,419, WO97/17100and WO 98/33443. Various deployment and/or collapsing arrangements areprovided for the umbrella filter. However, as the filter collapses, thecaptured embolic material tends to be squeezed outwardly towards an openend of the filter and pieces of embolic material may escape from thefilter with potentially catastrophic results. More usually, the filterumbrella is collapsed against the guidewire before removal through acatheter or the like. Again, as the filter membrane is collapsed, itwill tend to squeeze out the embolic material. Further, the umbrellafilter is generally fixed to the guidewire and any inadvertent movementof the guidewire during an interventional procedure can dislodge thefilter.

The insertion of such known filters in the human vasculature whichcomprises very small diameter blood vessels may result in inappropriatehaemodynamics which can exacerbate damage to the flowing blood and mayresult in haemolysis.

This invention is therefore directed towards providing an embolicprotection device which will overcome these major problems.

STATEMENTS OF INVENTION

According to the invention there is provided a collapsible filterelement for a transcatheter embolic protection device, the filterelement comprising:

-   -   a collapsible filter body which is movable between a collapsed        stored position for movement through a vascular system and an        expanded position for extension across a blood vessel such that        blood passing through the blood vessel is delivered through the        filter element;    -   a proximal inlet portion of the filter body having one or more        inlet openings sized to allow blood and embolic material enter        the filter body;    -   a distal outlet portion of the filter body having a plurality of        outlet openings sized to allow through-passage of blood, but to        retain embolic material within the filter body;    -   the distal outlet portion of the filter body in the region of        the outlet openings having means for reducing shear stress on        blood passing through the outlet openings.

In a preferred embodiment of the invention the shear stress reducingmeans includes lead-in radiussed portions of the filter body leading tothe outlet holes.

In a particular embodiment of the invention the shear stress reducingmeans includes lead-out radiussed portions of the filter body leadingfrom the outlet holes.

Most preferably the outlet-holes are generally circular.

In another preferred embodiment of the invention the proximal inletportion of the filter body in the region of the inlet openings has meansfor reducing shear stress on blood passing through the inlet openings.Preferably the shear stress reducing means includes lead-in radiussedportions of the filter body leading to the inlet holes. Ideally, theshear stress reducing means includes lead-out raduissed portions of thefilter body leading from the inlet holes.

In a particularly preferred embodiment the filter is of a polymericmaterial. Preferably the filter body defines a three dimensional matrix.Most preferably, the filter body is of a resilient elastomeric material.The filter body may be of a polyurethane elastomer. Most preferably thefilter body is of a polycarbonate urethane material.

In an especially preferred embodiment of the invention the filter bodyis covered with a hydrophilic coating, the openings being provided inthe coating.

Preferably the filter is of a polymeric material and the raduissedportions are formed by solvent polishing of the polymeric material.

In a preferred embodiment the porosity of the distal portion of thefilter body decreases towards the distal end of the filter. Ideally, theoverall porosity of the distal portion of the filter element is from 5%to 40%. Preferably the overall porosity of the distal portion of thefilter element is form 8% to 21%.

In a preferred embodiment in the transverse cross sectional areas atlongitudinally spaced-apart locations of the distal portion aresubstantially the same.

Preferably the distal portion is of generally conical shape having aradial dimension which decreases towards a distal end of the filterelement.

In one embodiment the distal portion includes a blind section adjacentto the distal end of the filter element. Preferably the blind portionextends longitudinally for at least 5% of the length of the distalportion, ideally for less than 30% of the length of the distal portion.

In a preferred arrangement the number of outlet holes increases towardsan outer edge of the distal outlet portion of the filter body.

Most preferably there are between 200 and 1000 outlet openings with anaverage diameter of between 50 and 200 microns. Ideally, there arebetween 200 and 300 outlet openings with an average diameter ofapproximately 150 microns. There may be at least 200 outlet openingswith an average diameter of no more than 200 microns.

Preferably there are less than 1000 openings with an average, diameterof at least 50 microns.

In a particularly preferred embodiment the openings are sized such thatshear stress imparted to blood flowing through the filter body atphysiological flow rates is less than 800 Pa, most preferably less thanabout 400 Pa and ideally less than about 200 Pa.

The openings are ideally generally circular openings.

In a preferred embodiment said filter body, when in a deployedconfiguration includes a generally cylindrical intermediate sectionbetween said proximal and distal portions. The filter body is generallytapered when in a deployed configuration. Preferably said distal sectionof said filter body comprises at least a portion of the filter element.Ideally said intermediate section of said filter body comprises at leasta portion of the filter element.

In a preferred embodiment the intermediate section of said filter bodyincludes a circumferential groove.

In a particularly preferred embodiment said filter body, when in adeployed configuration is defined by a generally elongated shape, havingan intermediate section with an axial dimension and a transversedimension, the ratio of the axial dimension to the transverse dimensionbeing at least 0.5, ideally at least 1.0.

In one embodiment of the invention the filter body includes a guidewirelumen extending co-axially of a longitudinal axis of the filter body.

In another aspect the invention provides a collapsible filter elementfor a transcatheter embolic protection device, the filter elementcomprising:

-   -   a collapsible filter body which is movable between a collapsed        stored position for movement through a vascular system and an        expanded position for extension across a blood vessel such that        blood passing through the blood vessel is delivered through the        filter element, the filter body having a proximal end, a        longitudinal axis and a distal end;    -   a proximal inlet portion of the filter body having one or more        inlet openings sized to allow blood and embolic material enter        the filter body;    -   a distal outlet portion of the filter body having a plurality of        outlet openings sized to allow through-passage of blood, but to        retain embolic material within the filter body;    -   the porosity of the distal portion of the filter body decreasing        towards the distal end of the filter.

In a further aspect the invention provides a collapsible filter elementfor a transcatheter embolic protection device, the filter elementcomprising:

-   -   a collapsible filter body which is movable between a collapsed        stored position for movement through a vascular system and an        expanded position for extension across a blood vessel such that        blood passing through the blood vessel is delivered through the        filter element;    -   a proximal inlet portion of the filter body having one or more        inlet openings sized to allow blood and embolic material enter        the filter body;    -   a distal outlet portion of the filter body having a plurality of        outlet openings sized to allow through-passage of blood, but to        retain embolic material within the filter body;    -   the filter body comprising a membrane of polymeric material;    -   wherein there are between 200 and 1000 outlet openings with an        average diameter of between 50 and 200 microns.

The invention also provides a collapsible filter element for atranscatheter embolic protection device, the filter element comprising:

-   -   a collapsible filter body which is movable between a collapsed        stored position for movement through a vascular system and an        expanded position for extension across a blood vessel such that        blood passing through the blood vessel is delivered through the        filter element;    -   a proximal inlet portion of the filter body having one or more        inlet openings sized to allow blood and embolic material enter        the filter body;    -   a distal outlet portion of the filter body having a plurality of        outlet openings sized to allow through-passage of blood, but to        retain embolic material within the filter body;    -   the filter body comprising a membrane of polymeric material;        wherein the openings are sized such that shear stress imparted        to blood flowing through the filter body at physiological flow        rates is less than 800 Pa, preferably less than about 400 Pa.

In a further aspect the invention provides a collapsible filter elementfor a transcatheter embolic protection device, the filter elementcomprising:

-   -   a collapsible filter body which is movable between a collapsed        stored position for movement through a vascular system and an        expanded position for extension across a blood vessel such that        blood passing through the blood vessel is delivered through the        filter element;    -   the filter body having a longitudinal axis a proximal inlet        portion, a distal outlet portion and an intermediate section        extending between the proximal portion and the distal portion;    -   a proximal inlet portion of the filter body having one or more        inlet openings sized to allow blood and embolic material enter        the filter body;    -   a distal outlet portion of the filter body having a plurality of        outlet openings sized to allow through-passage of blood, but to        retain embolic material within the filter body;    -   the filter body having a guidewire lumen co-axial with the        longitudinal axis;    -   wherein in a deployed configuration the intermediate section is        generally cylindrical with an axial dimension and a transverse        dimension, the ratio of the axial dimension to the transverse        dimension being at least 0.5, preferably at least 1.0.

In yet another aspect the invention provides a transcatheter embolicprotection device including:

-   -   a delivery system comprising:    -   a tubular member having a longitudinal axis, distal and proximal        portions, said distal portion of the tubular member being        removably advanceable into the vasculature of a patient;    -   a medical guidewire longitudinally axially movable in said        tubular member and having distal and proximal portions;    -   and a filter element of any aspect of the invention the filter        body having;        -   a first collapsed, insertion and withdrawal configuration an            a second expanded, deployed configuration;        -   a proximal inlet section and a distal outlet section, said            proximal inlet section including inlet openings which are            operable to admit body fluid when the filter body is in the            second expanded configuration;        -   a plurality of outlet openings disposed on at least a            portion of the filter element adjacent to the distal outlet            section;        -   wherein said filter body is moved between said first and            second configurations by displacement of said delivery            system.

Preferably the filter body has a collapsible filter frame operablycoupled thereto. Said frame may comprise a plurality of support armshaving proximal and distal ends. Preferably the arms are formed of anelastic shape memory material.

In preferred embodiment said frame is constructed such that filter bodyis biased toward said second, deployed configuration.

In one embodiment of the invention said inlet openings are defined atleast partially by said arms. Preferably proximal portions of said armsextend generally outwardly and distally from said guidewire when saidfilter body is in said second, deployed configuration.

In one embodiment distal portions of said arms extend generallyoutwardly and proximally from said guidewire when said filter body is insaid second, deployed configuration.

Preferably the distal portion of the tubular member further includes apod for receiving therein the filter body when in said first, collapsedconfiguration. Preferably said filter body is urged into said first,collapsed configuration by said pod when the guidewire is movedproximally.

In one embodiment said guidewire is solid.

In one arrangement said filter body comprises a sleeve slidably disposedon said guidewire. The device may further comprise stops for limitingthe range of longitudinal movement of the sleeve on said guidewire. Thesleeve may comprise a guidewire member distal to the filter bodytapering distally.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the followingdescription thereof given by way of example only with reference to theaccompanying drawings in which:

FIG. 1 is partially sectioned elevational view of an embolic protectiondevice according to the invention;

FIG. 2 is a schematic sectional elevational view of the embolicprotection device of FIG. 1;

FIG. 3 is a sectional view of the distal end of the device of FIG. 1shown in its loaded condition within its delivery catheter;

FIG. 4 is a longitudinal cross sectional view of the device of FIG. 1;

FIG. 5 is a cross sectional view of a distal end of the device of FIG.1;

FIG. 6 is a view on the line A-A in FIG. 4;

FIG. 7 is a perspective view of a filter body of the device of FIGS. 1to 6;

FIG. 8 is a side elevational view of the filter body of FIG. 7;

FIG. 9 is a view on a proximal end of the filter body;

FIG. 10 is a perspective view of a support frame;

FIG. 11 is a side elevational view of the support frame;

FIG. 12 is a perspective view illustrating the manufacture of thesupport frame;

FIG. 13 is a view of the support frame and filter body assembly;

FIGS. 14A to 14E are developed views of the distal end of a filter bodyillustrating different arrangements of outlet holes for filter sizes 6mm, 4 mm, 4.5 mm, 5 mm, and 5.5 mm respectively;

FIG. 15 is a side elevational view of another filter body of theinvention;

FIG. 16 is a developed view of the distal end of the filter body of FIG.15 illustrating an arrangement of outlet holes;

FIGS. 17( a) and 17(b) are perspective partially cut-away crosssectional views of a filter body before and after solvent polishingrespectively;

FIG. 18 is a graph of shear stress with outlet hole size and holenumber;

FIG. 19 is a longitudinal cross sectional view of a filter bodyaccording to the invention;

FIGS. 20 to 25 are longitudinal cross sectional views of differentembodiments of the filter body according to the invention;

FIGS. 26 to 28 are longitudinal cross sectional views of furtherembodiments of the filter body according to the invention;

FIG. 29 is a schematic perspective view of a filter element according toanother aspect of the invention;

FIGS. 30 to 33 are schematic perspective views of different embodimentsof the filter element according to the invention;

FIG. 34 is a schematic perspective view of a filter element according toa further aspect of the invention; and

FIGS. 35( a) to 35(d) are longitudinal side views of another filteraccording to the invention in different configurations of use.

DETAILED DESCRIPTION

Referring to FIGS. 1 to 13 there is illustrated an embolic protectiondevice as described in our WO-A-9923976 indicated generally by thereference number 100. The device 100 has a guidewire 101 with a proximalend 102 and a distal end 103. A tubular sleeve 104 is slidably mountedon the guidewire 101. A collapsible filter 105 is mounted on the sleeve104, the filter 105 being movable between a collapsed stored positionagainst the sleeve 104 and an expanded position as shown in the drawingsextended outwardly of the sleeve 104 for deployment in a blood vessel.

The sleeve 104 is slidable on the guidewire 101 between a pair ofspaced-apart end stops, namely an inner stop 106 and an outer stop whichin this case is formed by a spring tip 107 at the distal end 103 of theguidewire 101.

The filter 105 comprises a filter body 110 mounted over a collapsiblesupport frame 111. The filter body 110 is mounted to the sleeve 104 ateach end, the body 110 being rigidly attached to a proximal end 112 ofthe sleeve 104 and the body 110 being attached to a collar 115 which isslidable along a distal end 114 of the sleeve 104. Thus the distal endof the body 110 is longitudinally slidable along the sleeve 104. Thesupport frame 111 is also fixed at the proximal end 112 of the sleeve104. A distal end 116 of the support frame 111 is not attached to thesleeve 104 and is thus also free to move longitudinally along the sleeve104 to facilitate collapsing the support frame 111 against the sleeve104. The support frame 111 is such that it is naturally expanded asshown in the drawings and can be collapsed inwardly against the sleeve104 for loading in a catheter 118 or the like.

The filter body 110 has large proximal inlet openings 117 and smalldistal outlet openings 119. The proximal inlet openings 117 allow bloodand embolic material to enter the filter body 110, however, the distaloutlet openings 119 allow through passage of blood but retain undesiredembolic material within the filter body 110.

An olive guide 120 is mounted at a distal end of the sleeve 104 and hasa cylindrical central portion 121 with tapered ends 122, 123. The distalend 122 may be an arrowhead configuration for smooth transition betweenthe catheter and olive surfaces. The support frame 111 is shaped toprovide a circumferential groove 125 in the filter body 110. If thefilter 105 is too large for a vessel, the body 110 may crease and thisgroove 125 ensures any crease does not propagate along the filter 105.

Enlarged openings are provided at a proximal end of the filter body 110to allow ingress of blood and embolic material into an interior of thebody 110.

Referring in particular to FIGS. 10 to 13 the collapsible support frame111 has four foldable arms 290 which are collapsed for deployment andupon release extend outwardly to expand the filter body 110.

The support frame 111 can be manufactured from a range of metallic orpolymeric components such as a shape memory alloy like nitinol or ashape memory polymer or a shaped stainless steel or metal with similarproperties that will recover from the deformation sufficiently to causethe filter body 110 to open.

The support frame 111 may be formed as illustrated in FIG. 12 bymachining slots in a tube 291 of shape memory alloy such as nitinol. Onmachining, the unslotted distal end of the tube 291 forms a distalcollar 293 and the unslotted proximal end of the tube 291 forms aproximal collar 294. In use, as described above, the distal collar 293is slidably movable along the tubular sleeve 104 which in turn isslidably mounted on the guidewire 101 for deployment and retrieval. Theproximal collar 294 is fixed relative to the tubular sleeve 104.

To load the filter 105 the sub assembly of the support frame 111 andfilter body 110 is pulled back into the catheter 118 to engage thedistal stop 107. The support arms 290 are hinged inwardly and the distalcollar 293 moves forward along the tubular sleeve 104. As the supportarms 290 enter the catheter 118 the filter body 110 stretches as thefilter body collar 115 slides along the tubular sleeve 104 proximal tothe olive 120. On deployment, the catheter 118 is retracted proximallyalong the guidewire 101 initially bringing the collapsed filter assemblywith it until it engages the proximal stop 106. The catheter sleeve thenbegins to pull off the filter 105 freeing the support arms 290 to expandand the filter body 110 apposes the vessel wall.

For retrieval, a retrieval catheter is introduced by sliding it over theguidewire 101 until it is positioned at the proximal end of the filterbody 110 and support frame 111. Pulling the guidewire 101 will initiallyengage the distal stop 107 with the filter element and begin to pull itinto the retrieval catheter. The initial travel into the retrievalcatheter acts to close the proximal openings 117 of the filter element,thus entrapping the embolic load. As the filter 105 continues to bepulled back the filter body 110 and the support frame 111 are envelopedin the retrieval catheter. The collapsed filter 105 may then be removedfrom the patient.

Conveniently the tip of the catheter which forms a housing or pod forreception of the filter is of an elastic material which can radiallyexpand to accommodate the filter with the captured embolic material. Bycorrect choice of material, the same catheter or pod can be used todeploy and retrieve the filter. For deployment, the elastic materialholds the filter in a tightly collapsed position to minimise the size ofthe catheter tip or pod. Then, when retrieving the filter, the cathetertip or pod is sufficiently elastic to accommodate the extra bulk of thefilter due to the embolic material.

Also, the filter is not fast on the guidewire and thus accidentalmovement of the guidewire is accommodated without unintentionally movingthe filter, for example, during exchange of medical devices or whenchanging catheters.

It will also be noted that the filter according to the invention doesnot have a sharp outer edge as with many umbrella type filters. Rather,the generally tubular filter shape is more accommodating of the interiorwalls of blood vessels.

Conveniently also when the filter has been deployed in a blood vessel,the catheter can be removed leaving a bare guidewire proximal to thefilter for use with known devices such as balloon catheter and stentdevices upstream of the filter.

The outer filter body 110 is preferably of a resilient biocompatibleelastomeric material. The material may be a polyurethane based material.There are a series of commercially available polyurethane materials thatmay be suitable. These are typically based on polyether or polycarbonateor silicone macroglycols together with diisocyanate and a diol ordiamine or alkanolamine or water chain extender. Examples of these aredescribed in EP-A-461,375 and U.S. Pat. No. 5,621,065. In addition,polyurethane elastomers manufactured from polycarbonate polyols asdescribed in U.S. Pat. No. 5,254,622 (Szycher) are also suitable.

The filter material may also be a biostable polycarbonate urethanearticle an example of which may be prepared by reaction of anisocyanate, a chain extender and a polycarbonate copolymer polyol ofalkyl carbonates. This material is described in our WO 9924084.

The filter body may be manufactured from a block and cut into a desiredshape. The filter may be preferably formed by dipping a rod of desiredgeometry into a solution of the material which coats the rod. The rod isthen dissolved. The final geometry of the filter may be determined inthe dipping step or the final geometry may be achieved in a finishingoperation. Typically the finishing operations involve processes such asmechanical machining operations, laser machining or chemical machining.

The filter body is of hollow construction and may be formed as describedabove by dipping a rod in a solution of polymeric material to coat therod. The rod is then dissolved, leaving a hollow body polymericmaterial. The rod may be of an acrylic material which is dissolved by asuitable solvent such as acetone.

The polymeric body thus formed is machined to the shape illustrated inFIGS. 1 to 13. The final machined filter body comprises an inlet orproximal portion 210 with a proximal neck 212, and outlet or distalportion 213 with a distal neck 214, and an intermediate portion 215between the proximal and distal portions.

Alternatively the filter body may be formed by a blow moulding processusing a suitably shaped mould. This results in a filter body which hasthin walls.

The inlet holes 117 are provided in the proximal portion 210 which allowthe blood and embolic material to flow into the filter body. In thiscase the proximal portion 210 is of generally conical shape to maximisethe hole size.

The intermediate portion 215 is also hollow and in this case is ofgenerally cylindrical construction. This is important in ensuring morethan simple point contact with the surrounding blood vessel. Thecylindrical structure allows the filter body to come into soft contactwith the blood vessel to avoid damaging the vessel wall.

The intermediate portion 215 is provided with a radial stiffening means,in this case in the form of a radial strengthening ring or rim 220. Thering 220 provides localised stiffening of the filter body withoutstiffening the material in contact with the vessel. Such an arrangementprovides appropriate structural strength so that line apposition of thefilter body to the vessel wall is achieved. It is expected that othergeometrics of stiffening means will achieve a similar result.

The tubular intermediate portion 215 is also important in maintainingthe stability of the filter body in situ to retain captured emboli andto ensure that flow around the filter is minimised. For optimumstability we have found that the ratio of the axial length of theintermediate portion 215 of the filter body to the diameter of theintermediate portion 215 is preferably at least 0.5 and ideally greaterthan 1.0.

The outlet holes 119 are provided in the distal portion 213 which allowblood to pass and retain embolic material in the filter body.

The purpose of the filter is to remove larger particulate debris fromthe bloodstream during procedures such as angioplasty. In one case thefilter is used to prevent ingress of embolic material to the smallerblood vessels distal to a newly-deployed carotid stent. A known propertyof the filter is that it will present a resistance to the blood flow.The maximum blood pressure in the arterial system is determined by themuscular action of the heart. The cardiovascular system is amultiple-redundant network designed to supply oxygenated blood to thetissues of the body. The path from the heart through the site ofdeployment of the filter and back to the heart can be traced through thesystem. In the absence of the filter this system has a resistance, andthe flow through any part of it is determined by the distribution ofresistance and by the pressure generated by the heart.

The introduction of the filter adds a resistance on one of the paths inthe network, and therefore there will be a reduced blood flow throughthis part of the circuit. It is reasonable to assume that the flow alongthe restricted carotid will be inversely proportional to the resistanceof this branch of the circuit. For laminar flow in a tube the resistanceis independent of the flow rate.

The performance of vascular filters and particularly vascular filtersfor smaller blood vessels is determined by the relationship between thefilter and the media being filtered. Blood is a complex suspension ofdifferent cell types that react differently to different stimuli. Thedefining geometric attributes of the filter structure will establish thefilter's resistance to flow in any blood vessel. Ideally, all flow willbe through the filter and will be exposed to minimal damage.

All filters that do not have a sealing mechanism to divert flow onlythrough it and will have some element of flow around it. We haveconfigured the filter geometry such that flow through the filter ismaximised and flow around the filter is minimised. Pressure drop acrossthe face of the filter when related to the pressure drop through thealternate pathway will determine the filter efficiency.

Related to the pressure drop, is the shear stress experienced by theblood elements. Red cells have an ability to deform under the influenceof shear stresses. At low stresses (physiological) this deformation isrecoverable. Additionally, a percentage of the red cell population isfragile and will fragment at low shear stress even in patients with“healthy” cell populations. While the body can deal with the rupture andfragmentation of small numbers of red blood cells, gross red blood celldamage are likely to be problematic clinically. Consideration must begiven to the effects of the shear stresses, both the intensity andduration, on the constituent blood particles and the haemostaticmechanisms. It is the effects on the red blood cells and platelets thatare of primary importance.

Shear stresses can cause red cell destruction which is more pronouncedin patients with red cell disorders, such as sickle cell disease.Haemolysis can lead to anaemia, which can impede oxygen transportationaround the body, and in extreme cases causes damage to the kidneys, butthis would be unlikely given the relatively short duration of deploymentof vascular filters.

More importantly though, shear stress also causes damage to theplatelets themselves. Platelets play a key role in haemostasis and helporchestrate the complex cascade of events that lead to blood clotformation. The damage to the platelets causes communication chemicals tobe released, and these “activate” other platelets in the vicinity. Onceactivated, the platelets swell and their surfaces become sticky, andthis causes them to aggregate together and on available surfaces to forma “clump”. The released chemicals attract and activate other plateletsin the area such that the clump grows in size. Fibrous proteins are alsocreated and together a blood clot (thrombus) is formed. Depending on itssize and position, the thrombus may occlude some of the holes in avascular filter. It is also possible for the thrombus to becomedetached, particularly on removal of the device, and float freely awaydownstream to become an embolus. Should the embolus be large enough tobecome trapped in a narrow arterial vessel further along the system,flow in that vessel would be compromised and this could lead directly tostroke. Platelet aggregation occurs most effectively in stagnant andre-circulating flow regions.

It is also known that activated platelets can coat foreign bodies in theblood, such as intravasculature catheters. The foreign material surfacethen becomes sticky and therefore a site for further aggregation. Thisin turn could affect the local geometry of the device and the local flowcharacteristics.

Shear may be expressed as follows:

-   -   Wall shear stress: τ=4 μQ/πR³    -   Where        -   μ is the blood viscosity        -   Q is the mass flow rate        -   R is the vessel radius

In FIG. 18 we show the relationship under specific flow conditions in astated diameter of vessel. This plot assumes a Newtonian fluid, equalflow rate through each hole, a flow rate of 270 ml/min and a 4 mm bloodvessel.

The relationship shows that as hole size decreases, then the requirednumber of holes increases significantly.

This representation of shear is a good general representation however,local conditions at the filter pores can have significant impact on theshear with flow irregularities generating the possibility of shearlevels increasing by an order of magnitude. The location of the maximumshear stress is at the edges of the filter holes at their downstreamside. The filter element of the invention has local radii and the filterentrance and exit holes to minimise the shear stress levels. Holes maybe drilled using mechanical drilling or laser cutting. However, theseprocesses can produce dimensionally repeatable holes but will impartsurface conditions that are not suitable for small vessel filtration.Any fraying of edges due to mechanical cutting will certainly cause flowdisruptions and form sites for platelet aggregation. Similarly lasercutting due to its local intense heating and vaporisation of thesubstrate will lead to pitting, surface inclusions, rough edges andsurface imperfections.

In the invention the holes are post processed to modify the surfaces andto radius the edges. A preferred embodiment of the filter element ismanufactured using a medial grade polyurethane such as Chronoflex™supplied by Cardiotech Inc. The filter holes are post-processed bysolvent polishing using acetone or other suitable solvent.

Referring in particular to FIG. 17( a) there is illustrated a section ofa polymeric filter body with a number of machined outlet holes 119.After solvent polishing the hoes are surface treated providing radiusedlead-in and lead-out portions.

Solvent polishing of the membrane is achieved by softening the materialin the surface layers of the membrane such that a local reflow processis facilitated. This reflow is achieved using one of two classes ofsolvent.

-   -   Solvents that have an ability to dissolve the polymer.    -   Solvents that have an ability to swell the polymer.

The process for the first class of solvents involves exposing themembrane to a limited amount of the solvent. This is achieved by dippingthe membrane in the solvent for a short time or exposing the membrane toconcentrated vapours of the solvent for a time. The solvent is absorbedinto the surface layers and they become solubilised. The solubilisedsurface layers act like a viscous liquid and they adopt configurationsof lowest surface energy. The lowest energy configuration for a liquidis a sphere. The sharp edges and corners become rounded by thesolubilisation of the surface. The solvent is dried to reveal a smoothsolvent polished surface.

Swelling solvents act slightly differently in that they cannot dissolvethe material. However their ability to swell the material allows similarreflow processes to occur. The key difference is that the membrane isimmersed in the solvent for a longer period of time, preferably inexcess of 30 minutes. The solvent swelling process is most effectivewhen the membrane material is a two phase polymer such as a polyuerthaneor a PEBAX, as the solvent can be selected to match either phase.

Solvents will dissolve polymers when their solubility parameters aresimilar. Solvents will swell a polymer when their solubility parametersare slightly different. Preferably the swelling solvent swells thematerial by less than 30%. Above this level the solvent should beconsidered dissolving solvent.

Having reduced the local shear stresses as described above, it is thendesirable to minimise the propensity for the activated platelets toadhere to the filter substrate. The more preferred embodiment of filteris one where the polished polymeric surface is combined with a coatingon the substrate.

The swelling of the polymer matrix reduces residual stresses that mayhave developed during the coated core drying or lasering processes.During the lasering process, the material in the immediate proximity ofthe lasered holes will have been exposed to heat. This heat will disrupthard segment crystallites and they will reform to lower ordermeta-stable structures or be completely dissolved in the soft phase. Theheat will also induce the soft segments to contract, however, there-arrangement of the hard segments imposes new restrictions on therecovery of the soft segments to an equilibrium (relaxed) state. Thus,on removal of the heat source (laser), the morphology of the blockcopolymer will have changed, in the sense that the new configurations ofthe hard segments and soft segments will have been frozen in. Afterlasering, the holes have sharp and well-defined geometries. Afterexposing the coated material to the solvent, the solvent uncoils thesoft segment chains and disassociates low ordered hard segment that aredissolved in the soft segment phase, so on removal of the solvent, thepolymer matrix dries in a more relaxed state. In so doing, the sharp,well-defined walls of the lasered holes are transformed to a morecontoured relaxed state.

Such applicable solvents for this application, but not limited to, are2-propanone, methyl ethyl ketone or trichloroethylene.

The solvent characteristics are described as follows at roomtemperature:

-   -   The solvent is organic, colourless and in a liquid state.    -   The overall solubility parameter of the solvent is quoted        between 16 to 26 Mpa^(0.5).    -   The solvent is polar and is also capable of hydrogen bond        interactions.    -   On partitioning the overall solubility parameter of the solvent        into dispersion, polar and hydrogen bonding components, the        hydrogen bonding value (in its own solution) is quoted between 3        Mpa^(0.5) to 8.5 Mpa^(0.5)    -   The solvent is infinitely misible in water.    -   The solvent is aprotic (proton acceptor) towards the formation        of hydrogen bonding between it and the polymer.

We have found that the optimum average diameter of the outlet holes inthe polymeric membrane is from 100 to 200 microns, ideally approximately150microns. The number of holes in the distal portion 213 is from 200 to500, ideally about 300. This hole size and number of holes minimisesshear levels by reducing localised flow rates. Thus, we have found thatshear can be maintained below 800, preferably below 500 and ideallybelow 200 Pa at a blood flow rate of up to 270 ml/min in a 4 mm bloodvessel. Ideally the holes are circular holes.

We have found that by maintaining blood shear below 800, preferablybelow 500 and ideally below 200 Pa, the filter provides appropriatehaemodynamics to minimise turbulence and inappropriate shear stress onnative arteries and veins. Damage to flowing blood such as haemolysiswhich involves the destruction of red blood cells by rupture of the cellenvelope and release of contained haemoglobin is avoided. The outlethole size and number of holes is optimised in order to capture embolicmaterial, to allow the embolic material to be entrapped in the filterbody and to be withdrawn through a delivery device such as a deliverycatheter on collapsing of the filter body.

Shearing of red blood and damage to platelets during filtration is aproblem easily solved in extra-corporeal circuits by providing largefilter areas with consequent low flow rates through individual porescontrolled to flow rates such that the shear is maintained in rangesthat are below known threshold levels with clinical relevance.

However, as shear stress increases in inverse proportion to the cube ofthe radius, small blood vessels do not provide space in which to controlshear levels by reducing localised flow rates. At flow rates up to 270ml/min in a 4 mm blood vessel we have found that we can maintain shearat levels below 200 Pa with 150 micron holes.

We have also found that the porosity of the distal end of the filtermembrane and the arrangement of outlet holes is important in optimisingcapture of embolic material without adversely effecting blood shearcharacteristics and the material properties of the filter body whichallow it to be collapsed for delivery, expanded for deployment andcollapsed for retrieval.

Referring in particular to FIGS. 7, 8 and especially 14 (a) to 14 (e) wehave found that the overall porosity of the filter element is preferablybetween 5% and 40% and ideally between 8% and 21%. The transverse crosssectional areas of the filter body at longitudinally spaced-apartlocations of the distal portion are substantially the same. Mostimportantly we have found that the porosity of the distal portion of thefilter body should decrease towards the distal end. Arrangements ofdistal holes 119 for different filter diameters are shown in FIGS. 14(a) to 14 (e). FIG. 14 (a) shows an arrangement for a 6 mm filter, 14(b) for a 4 mm filter, FIG. 14 (c) for a 4.5 mm filter, FIG. 14 (d) fora 5 mm filter and FIG. 14 (e) for a 5.5 mm filter. The number of outletholes 119 also increases towards an outer edge of the distal portion ofthe filter body.

In addition we have found that for optimum capture of embolic materialwhile facilitating retrieval of the filter with entrapped embolicmaterial into a retrieval catheter the distal portion of the filterelement includes a blind section 130 adjacent the distal end of thefilter element. Ideally the blind portion 130 extends longitudinally forat least 5% and preferably less than 30% of the length of the distalportion.

In order to reduce the profile of the filter body we have significantlyreduced the thickness of the filter membrane to typically in the orderof 25 microns. This reduction in thickness however means that themembrane used must have a relatively high stiffness to achieve acomparable strength. However, we have found that such an increase instiffness results in poor memory performance and is thereforeundesirable.

We have surprisingly found that by providing a filter body of laminateconstruction in which a membrane is coated with a coating to a thicknessof from 5% to 40% of the thickness of the membrane we have been able toprovide a filter body which has a low profile but which has good memorycharacteristics.

In particular, we have found that hydrophilic coatings and hydrogels arehighly suitable coatings as they have a similar surface to theendothelial lining of a blood vessel and are not perceived by the body'simmune system as foreign. This results in at least reduction and in somecases substantial elimination of platelet adhesion and fibrin build upwhich could otherwise occlude the filter and/or create a harmfulthrombus. The coating also provide a relatively low friction surfacebetween the filter body and the delicate endothelial lining of a vesselwall and therefore minimise the trauma and injury to a vessel wallcaused by deployment of the filter body in the vasculature.

A hydrogel will absorb water swelling its volume. The swelling of thehydrogel will exert an expansion force on the membrane helping to pullit into its recovered or deployed shape.

A coating that expands on contact with blood will exert an expansionforce on the membrane helping to pull it into its recovered or deployedshape.

A coating that expands when subjected to body temperature will exert anexpansion force on the membrane helping to pull it into its recovered ordeployed shape.

Hydrophilic coatings can be classified by their molecular structure:

-   -   Linear Hydrophilic polymers can dissolve or be dispersed in        water    -   Cross-linked hydrophilic polymers, which include hydogels, can        swell and retain water.

Hydrophilic coatings may be also synthetic or natural. Synthetichydrophilic polymers include the following:

-   -   Poly (2-hydroxy ethyl methacrylate)—(PHEMA)    -   Poly (vinyl alcohol)—(PVA)    -   Poly (ethylene oxide)—(PEO)    -   Poly (carboxylic acids) including:    -   Poly (acrylic acid)—(PAA)    -   Poly (methacrylic acid)—(PMAA)    -   Poly (N-vinyl-2-pyrollidone)—(PNVP)    -   Poly (sulfonic acids), poly (acrylonitrile), poly (acrylamides)

Natural hydrophylics include:

-   -   Cellulose ethers    -   Collagen    -   Carrageenan

Commercially available hydrophylic coatings suitable for coating filtermembrane include, but are not limited to the following:

-   -   Aquamer (Sky Polymers Inc.)    -   Phosphorylcholine (PC) (Biocompatibiles Ltd)    -   Surmodics (Surmodics Inc. BSI)    -   Hydak (Biocoat Inc)    -   Hydomer (Hydormer Inc)

Hydrogels as stated are cross-linked hydrophilic molecules. Themolecular mobility of hydrogels is constant and extensive, givingceaseless molecular motion, which contributes to the property ofbiocompatibility by inhibiting protein absorption.

The extent to which a hydrogel imparts properties of biocompatibility,wettability and lubricity is directly related to the amount of water itabsorbs into its molecular matrix, which is referred to as the “degreeof swelling”.W=[(Wsw−Wo)/Wsw]×100

-   -   Where Wsw=Weight of swollen gel        -   Wo=Weight of dry gel            Water uptake=U=[(Wsw−Wo)/Wsw]×100

A typical hydrogel will absorb up to 20% of their dry weight of water.Superabsorbant hydrogels will absorb up to 2000% of their dry weight ofwater.

Hydrogel strength is directly related to cross link density (μ) andmolecular weight between cross-links (Mc).

Hydrophilic coatings may be typically applied by dipping, sprayingand/or brushing. The coatings may also be applied by solution or bycolloidal dispersion.

The membrane surface to be coated may be prepared by cleaning with asolvent and/or ultrasonic cleaning. Plasma or corona discharge may alsobe used to increase the surface energy and thus provide for betteradhesion.

Alternatives to Hydrophilics include low friction fluoropolymer, i.e.PTFE & FEP coatings that are chemically inert and have low coefficientsof friction, which also helps prevent adhesion of platelets.

Other coatings that rely on being chemically inert include.

-   -   Poly-para-xylylene (Paralene N, C & D) made by Novatron Limited.    -   Diamond like carbon.    -   TetraCarbon (Medisyn Technologies Ltd.).

Both diamond like carbon & tetracarbon also provide very thin hardsurface layers, which help reduce the dynamic coefficient of frictionfor elastomers.

The coating may be typically applied by dipping, spraying and/orbrushing. The coatings may also be applied by solution or colloidaldispersion.

Typically, to produce a filter according to the invention a polymericfilter membrane is first produced by machining a core of a desired shapefrom an inert material such as perspex. The perspex core is then dippedin a solution of a polymeric material as described above. Alternativelythe membrane is formed by blow moulding. Holes are then laser machinedin the dipped core. The perspex core is removed by dissolving inacetone. Residual acetone is washed out with water.

A filter frame of gold plated Nitinol is mounted on a filter carrier inthe form of a polyimide tube. The filter membrane is then slid over thefilter support frame to provide an uncoated filter assembly.

The filter assembly is dipped in a solvent such as propan 2-ol to cleanthe assembly. The cleaned assembly is then dipped in a solution of acoating material. A vacuum is applied to remove excess coating materialprior to drying in an oven. The coating material is typically of Aquamerin a water/ethanol solution. The thickness of the coating is typically 2to 10 microns.

Preferably the filter body contains regions of varying stiffness anddurometer hardness. The change in filter stiffness along its geometrycan be achieved by varying the material properties or by modificationsto the thickness or geometry of the membrane. The change in materialhardness is achieved by varying the material properties. The polymermaterial may be one of the following: polyamides, polyurethanes,polyesters, a polyether block amide (PEBAX), olefinic elastomer,styrenic elastomer. Ideally the filter body has a durometer of between60 D and 70 A Shore hardness

Referring to FIG. 19 there is illustrated a filter element comprising afilter body 2 according to the invention. In this case, the filter body2 has a proximal section 3 and a distal section 4 interconnected by anintermediate section 5. Both the proximal section 3 and the distalsection 4 are made from a relatively stiff grade of polyurethanematerial which enables a low wall thickness to be achieved, thusadvantageously minimising the bulk of the filter when it is in acollapsed position so that it has a low crossing profile while at thesame time providing adequate strength. The intermediate section 5 ismade from a soft elastic grade of polyurethane having good shape memorycharacteristics which will help the filter maintain the desired expandedshape during use of the filter. This soft portion also allows one filtersize to accommodate a range of vessel sizes conforming closely to thevessel wall to prevent blood and embolic material bypassing the filter.

In the filter body 2 illustrated in FIG. 19 the body is of generallyuniform thickness in cross section. However, to achieve any desiredvariation in the properties of the filter body the thickness may bevariable such as in the filter body 10 illustrated in FIG. 20.

Referring to FIGS. 21 to 25, any required structural properties may alsobe provided by a filter body, which is at least partially of a laminateconstruction. The layers of the laminate may be of the same or differentmaterials. In the illustration of FIG. 21 the distal section 4 and partof the intermediate section 5 are of a two layer 21, 22 construction.The layers 21, 22 may be of the same or different materials.

The layers 21, 22 are keyed together by mechanical or chemical means,the holes in the distal section 4 are then formed by boring through thetwo layers 21, 22.

In the illustration of FIG. 22 the entire filter body 30 is of a threelayer 31, 32, 33 construction. Layer 31 is a structural layer made froma material such as polyether block amide (PEBAX), polyester,polyethylene, polyurethane, terephthalate (PET), or nylon. Layers 32, 33are coating layers made from a material such as a hydrophilic, hydrogel,non-thrombogenic, or non-stick material. Layers 32, 33 may be of thesame or different materials. The holes at the distal end 4 are alsolined with the coating layers 32, 33.

When coating layers 32, 33 are of different materials, they are appliedto structural layer 31 as follows. A temporary protective film is firstsealed to the outer most surface of layer 31. Then coating layer 33 isapplied to the inner most surface of layer 31 by immersing the bodyformed by layer 31 in a coating solution. Excess coating solution issucked out and the protective film is removed from the outer mostsurface of layer 31. Another temporary protective film is then sealed tothe inner most surface of layer 33. The body formed by layers 31, 33 iscompletely immersed in a coating solution. Excess coating solution isdrawn out and the protective film is removed from the innermost surfaceof layer 33.

If the coating layers 32, 33 are of the same material, both layers 32,33 may be applied to the structural layer 31 in one step without the useof protective films.

In the illustration of FIG. 23 the entire filter body 45 is of a threelayer 46, 47, 48 construction. Layers 46, 47, 48 are structural layersand layers 47, 48 are of the same material. The holes at the distal end4 are also lined with the structural layers 47, 48.

In the illustration of FIG. 24 the entire filter body 50 is of a threelayer 51, 52, 53 construction. Layers 51, 52, 53 are structural layers,and in this embodiment layers 52, 53 are of different materials.

In the illustration of FIG. 25 the entire filter body 55 is of a fourlayer 56, 57, 58, 59 construction. Layers 56, 57 are structural layersand may be of the same or different materials. Layers 58, 59 are coatinglayers and may be of the same or different materials. The holes at thedistal end 4 are also lined with the coating layers 58, 59.

Referring to FIG. 26 there is illustrated another filter element 60according to the invention, which is similar to part of the distalsection 4 of filter element 2 of FIG. 19. But having no proximal webbingmembers thus maximising the size of the inlet opening.

FIG. 27 illustrates a filter element 61, which is similar to the distalsection 4 and part of the intermediate section 5 of filter element 20 ofFIG. 21, having the advantages of the laminate structure previouslydescribed, combined with the large inlet opening of FIG. 26 and thevariable distal geometry of FIG. 19 (enabling the filter to accommodatea range of vessel sizes).

FIG. 28 illustrates a further filter element 65, which includes asupport ring 66 to maintain the intermediate section 5 open to advancingblood flow. Support ring 66 may be arranged perpendicular to thedirection of the blood flow or inclined at an angle, as illustrated inFIG. 28. The support ring 66 may be of an elastic, super elastic orshape memory material, and may be either actuated remotely to appose thevessel wall in a perpendicular or close to perpendicular position, orfixed in circumference so that its inclination and shape are controlledby the diameter of the vessel.

A different layer structure may be provided at any desired location ofthe filter body to achieve required properties.

Referring now to FIG. 29 there is shown another filter element accordingto the invention, indicated generally by the reference 70. The filterelement 70 has a filter body 72 of generally similar construction to thefilter element described previously, the body having a proximal section73 and a distal section 74 interconnected by an intermediate section 75.In this case, the distal section 74 is of a relatively hard polyurethanematerial whilst the proximal section 73 and intermediate section 75 areof a softer grade polyurethane material. A number of longitudinal ribs76 are provided around a circumference of the proximal section 73.Advantageously, this construction facilitates close engagement of anouter circumference of the proximal section 73 against a vessel wall tominimise the risk of embolic material bypassing the filter element 70.An internal support frame, as described above, urges the proximalsection 73 outwardly so that it expands against and closely conformswith the wall of the blood vessel in which the filter element 70 ismounted in use.

Conveniently, the corrugations or ribs 76 allow the proximal section 73of the filter element 70 to accommodate a wider range of vessel sizeswhilst maintaining good contact between the outer circumference of theproximal section 73 and the vessel wall and providing improved filterbody integrity.

Referring to FIG. 30 there is illustrated another filter element 80according to the invention. In this case corrugations 81 are providedfor improved filter body integrity.

Referring to FIG. 31 there is illustrated another filter element 82according to the invention. In this case the cross section of the filterelement 82 is of a flower petal shape with a plurality of longitudinallyextending ribs 83 for improved apposition. As explained in reference toFIG. 29, the “petal shaped” cross section (as for corrugations) increasethe circumference of the filter body, thus enabling the body to beapposed closely against the vessel wall by a supporting structure in awide range of vessel sizes.

Referring to FIG. 32 there is illustrated another filter element 85according to the invention. In this case slits 86 are provided in theplace of the corrugations or “petal shapes” shown above. The slits 86enable the body of the filter to conform to a range of vessel diamtersby overlapping and preventing creasing in small diamater vessels, orallowing the body to expand with the aid of a supporting structure inlarger diameter vessels. In both instances close engagement of the outercircumference with the vessel wall is facilitated, thus minimizing therisk of embolic material bypassing the filter.

Referring to FIG. 33 there is illustrated another filter element 88according to the invention. In this case ribs 89 are provided to preventcreases forming along the filter element 88 in the longitudinaldirection, and also to allow expansion of the filter element 88.

Referring to FIG. 34 there is illustrated a further filter element 90according to the invention, which is of a concertina-like shape with twocircumferentially extending grooves 91, 92. This circumferential groovesor ribs have several advantages. They add to the integrity of the filterbody, assisting it in maintaining its shape in the vessel afterdeployment. They inhibit the propagation of creases between the varyingdiameter body segments, so that one filter can be designed for a rangeof vessel sizes. They enable the filter to extend in length to greatlyincrease its effective volume without adding to the length of thedeployed device in use. This provides the benefit of safe retrieval oflarge embolic loads as explained with reference to stretchable membranesbelow.

Referring to FIGS. 35( a) to 35(d) there is illustrated another embolicprotection system according to the invention incorporating a filterelement 94 according to the invention which is similar to thosedescribed above. The protection system includes a guidewire 95 and aretrieval catheter 96 which is advanced over the guidewire to retrievethe filter containing trapped embolic material 97. In this case thefilter body includes an intermediate 98 and distal 99 membrane, one orboth of which are stretchable to facilitate the retrieval of thecaptured embolic material 97. The stretching of the membrane during theretrieval process is illustrated in FIGS. 35( b) to 35(d).

The use of such a stretchable filter membrane allows larger volumes ofcaptured embolic material to be retrieved than would be possible with astiffer membrane. This is possible because if a filter is to beretrieved by withdrawing it into or through a catheter of a giveninternal diameter, the maximum volume of material that can be retrievedis directly proportional to the length of the filter and the internaldiameter of the catheter. The stretchable membrane allows the filter toincrease in length upon retrieval, thus increasing the space availablefor retention of captured embolic material. This is particularlysignificant in the case of large volumes of captured embolic material,which will be more difficult to safely retrieve with a non-stretchabledevice.

The stretchable section may include some or all of the filter body, andmay not necessarily include the distal cone. The distal cone containingthe outlet pores may be formed from a non stretch material, while theinter mediate filter body is stretchable. This provides the advantage offilter extension during retrieval while preventing the problem ofrelease of captured material through expanding distal pores.

Another advantage of the stretchable section is that the crossingprofile can be reduced as the filter can be loaded into a delivery podin a stretched, rather than bunched or folded, configuration. Thisreduces the volume of filter material contained in any given crosssection of the loaded delivery pod.

In addition the use of a stretchable filter material in the intermediatesection can also be advantageous by providing a section of the filterbody which can be circumferentially expanded by a support frame toappose the wall of a wide range of vessel sizes.

The invention is not limited to the embodiments hereinbefore describedwhich may be varied in detail.

1. An embolic protection device, comprising: a filter membrane of apolymeric material disposed over a collapsible support frame, thesupport frame and filter membrane at least partially movable between acollapsed position and a deployed position; and a coating applied to thefilter membrane to reduce thrombus formation in the presence of bloodflowing through the embolic protection device, wherein the filtermembrane comprises at least 200 outlet openings with an average diameterof from 100 to 200 microns, and the shear stress imparted to bloodflowing through the embolic protection device at physiological flowrates is less than 800 Pa.
 2. The embolic protection device according toclaim 1, wherein the coating has a thickness between about 5% and 40% ofthe thickness of the filter membrane.
 3. The embolic protection deviceaccording to claim 1, wherein the coating is a cross-linked hydrophilicpolymer.
 4. The embolic protection device according to claim 1, whereinthe coating is a synthetic hydrophilic polymer.
 5. The embolicprotection device according to claim 1, wherein the coating is a naturalhydrophilic polymer.
 6. The embolic protection device according to claim2, wherein the coating is a hydrogel.
 7. The embolic protection deviceaccording to claim 6, wherein the hydrogel is configured to absorbwater, thereby sealing the filter membrane to a body lumen.
 8. Theembolic protection device according to claim 1, wherein the coating isconfigured to mimic an endothelial lining of a membrane lumen.
 9. Theembolic protection device according to claim 1, wherein the coating isconfigured to provide a low friction surface interaction between thefilter body and a wall of a body lumen.
 10. The embolic protectiondevice according to claim 1, wherein the filter membrane is treated toincrease surface energy of the filter membrane prior to being coated.11. The embolic protection device according to claim 1, wherein thefilter membrane comprises a proximal portion, an intermediate portionand a distal portion, and the distal portion of the filter membranecomprises a blind section adjacent to the distal end of the distalportion, wherein the blind section extends longitudinally over 5% to 30%of the length of the distal portion of the filter membrane and there areno outlet openings in the blind section.
 12. The embolic protectiondevice according to claim 11, wherein in the deployed position, theintermediate portion is generally of a cylindrical shape and the distalportion is of a generally frusto-conical shape tapering distallyinwardly.
 13. The embolic protection device according to claim 11,wherein the filter membrane comprises between 200 and 1000 outletopenings.
 14. The embolic protection device according to claim 11,wherein the number of outlet holes per unit area increases in theproximal direction beginning at a proximal end of the blind section. 15.The embolic protection device according to claim 1, wherein the numberof outlet holes per unit area increases distally to proximally.
 16. Theembolic protection device according to claim 1, wherein the filtermembrane has at least 800 outlet openings.
 17. The embolic protectiondevice according to claim 16, wherein the outlet holes have an averagediameter of from 100 to 150 microns.
 18. The embolic protection deviceaccording to claim 1, wherein the outlet holes have an average diameterof from 100 to 150 microns.
 19. The embolic protection device accordingto claim 1, wherein the shear stress imparted to blood flowing throughthe embolic protection device at physiological flow rates is less than500 Pa.
 20. The embolic protection device according to claim 1, whereinthe shear stress imparted to blood flowing through the embolicprotection device at physiological flow rates is less than 200 Pa. 21.An embolic protection device comprising: a collapsible filter body whichis movable between a collapsed stored position for movement through alumen and an expanded position; a filter membrane disposed at leastpartially over a portion of the filter body; a proximal inlet portion ofthe filter membrane having one or more inlet openings sized to allowfluid and debris to enter into the filter body and membrane; a distaloutlet portion of the filter membrane having at least 800 outletopenings with an average diameter from 100 to 150 microns formed thereinto allow fluid to pass therethrough while maintaining debris within thefilter membrane; and a coating applied to the filter membrane, thecoating configured to reduce shear stress on blood passing through andaround the expanded filter body and membrane, wherein the distal outletportion of the filter membrane comprises a blind section adjacent to thedistal end of the distal portion, the blind section extendslongitudinally over 5% to 30% of the length of the distal outlet portionof the filter membrane, there are no outlet openings in the blindsection, and the shear stress imparted to blood flowing through theembolic protection device at physiological flow rates is less than 800Pa.
 22. The embolic protection device according to claim 21, wherein thecoating has a thickness between about 5% and 40% of the thickness of thefilter membrane.
 23. The embolic protection device according to claim21, wherein the number of outlet holes per unit area increases distallyto proximally beginning at a proximal end of the blind section.
 24. Theembolic protection device according to claim 21, wherein the shearstress imparted to blood flowing through the embolic protection deviceat physiological flow rates is less than 500 Pa.
 25. The embolicprotection device according to claim 21, wherein the shear stressimparted to blood flowing through the embolic protection device atphysiological flow rates is less than 200 Pa.
 26. An embolic protectiondevice, comprising: a filter membrane of a polymeric material disposedover a collapsible support frame, the support frame and filter membranemovable between a collapsed position and a deployed position, whereinthe filter membrane comprises at least 200 outlet openings having anaverage diameter of from 100 to 150 microns, and the shear stressimparted to blood flowing through the embolic protection device atphysiological flow rates is less than 200 Pa.